1. Field of the Invention
This invention relates generally to apparatus for forming encapsulated semiconductor chips and, more specifically, to an automatic plunger apparatus for compressing molten plastic into a cavity for forming an encapsulated semiconductor device.
2. Description of the Related Art
Automatic plunger apparatus for use in encapsulating semiconductor devices are well known to those skilled in the art. In general, semiconductor chips coupled to a leadframe are contained within a cavity. Plastic pellets are heated in order to form a molten plastic fluid which is compressed by the plunger to fill the cavity with the molten plastic fluid. Occasionally, the force exerted by this compression is substantial enough to damage the plunger, the semiconductor chip, and/or other components.
Accordingly, automatic plunger apparatus are typically designed to have over-pressure protection. For example, in the prior art automatic plunger apparatus, the pressure in the cavity is sensed electronically in order to create an electrical signal corresponding to the measured pressure inside the cavity. This signal is compared using an electronic comparing circuit having a signal representation corresponding to the maximum pressure permitted within the cavity. If the signal corresponding to the measured pressure exceeds the signal corresponding to the maximum pressure permitted within the cavity, then the comparing circuit generates control signals to stop compressing the molten plastic fluid. This prior art approach has several disadvantages. Specifically, components within the electronic control system sometimes fail, thereby causing a temporary loss of over-pressure protection. Additionally, the prior art approach to provide over-pressure protection only checks for the pressure within the cavity. It is possible that the plunger could be mechanically jammed such that there is no compression of the molten fluid, and therefore, no indication of a problem. Nonetheless, in this case, the plunger can be severely damaged.
Therefore, there existed a need to create a more reliable automatic plunger apparatus that disengages the applied force to the plunger when the plunger is prevented from compressing the molten plastic into the cavity.